This is one of several Puerto Rican side dishes that can be described as "rice with stuff in it". In particular, this one is served alongside the pork at the lechoneras in Guavate. One of my favorite bites we had in PR was the takeout container of this we brought with us to the airport to eat before our flight home.
I don't have a whole lot of specific measurements here, but whatever you do will probably work.
Pigeon peas can be found among the Mexican/Hispanic ingredients at your local well-stocked megamart. So can annatto seeds; to make achiote oil, combine 1/4 cup of the seeds with 1/2 cup of corn or canola oil, simmer for 10 minutes, then strain when it cools. It will look like yellow food coloring, and will impart a beautiful yellow color to whatever you cook in it along with a subtle flavor. (It's known as the "poor man's saffron".) You can buy a sazon that's mostly achiote and coriander that's more traditional, but since coriander is generally frowned upon in my household, I use the regular Goya adobo seasoning.
Arroz con Gandules
1 pound medium-grain rice
2 cans green pigeon peas (gandules)
2 onions, diced
5 cloves garlic, minced
1 bell pepper, diced
1/2 cup olives, chopped
1-16 oz can tomatoes, drained
roast pork, diced (however much you have)
Heat the achiote oil in a skillet over medium-low heat and add all of the ingredients except for the rice, pork, and gandules. Saute slowly for about 30 minutes, until it all sort of collapses into a cohesive mass (known as a sofrito).
Meanwhile, rinse your rice until the rinsing water is clear. Put in a pot and cover with water to a depth of one knuckle over the rice. Bring to a boil, then add the sofrito, pork, and pigeon peas. Cover, reduce to the lowest possible heat, and let sit for about 20 minutes or until it's done. Stir and add more adobo if it needs it.